Retrieve a billing alert 

{
  "id": "alrt_12345",
  "object": "billing.alert",
  "title": "API Request usage alert",
  "livemode": true,
  "alert_type": "usage_threshold",
  "usage_threshold": {
    "gte": 10000,
    "meter": "mtr_12345",
    "recurrence": "one_time"
  },
  "status": "active"
}